The present invention relates to improvements for a rock picking machine originally designed by John Schulte, as described in Canadian Patent 699,748. The Schulte machine is subject to "jamming" by rocks and soil retention under certain conditions. Some of these conditions are outlined in Evaluation Report Number E3880B published by the Prairie Agricultural Machinery Institute (PAMI) in September of 1981. The PAMI Evaluation Report on page 4 identifies a "jamming" problem with respect to large rocks; "Rocks larger than 520 mm (20 in.) would not pass between the conveyer crossmember and the grate." In order to avoid problems of soil retention, the PAMI Evaluation Report made recommendations with respect to operating depth and field preparation. The suggested operating depth was "with the grate just touching the soil surface". The short comings of this suggested operating depth was described on page 4 as follows; "This was adequate for removing rocks lying on the surface, however, partially buried rocks were pushed back into the soil by the grate." The suggested field preparation for best performance was " in fields with a firm base and a minimum amount of trash or dirt lumps." The Applicant has found that operating at the recommended operation depth sometimes results in rocks jamming between the conveyer teeth and the forward edge of the grate. The PAMI Evaluation Report also indicated some "jamming" which occurred with small rocks. In most cases these jammed rocks could be removed by reversing the conveyer. However, small rocks "sometimes jammed between the conveyer bat teeth and the rear of the grate, preventing the conveyer from being reversed". Another type of "jamming" noted by the Applicant is by rocks which, instead of remaining in the bucket, are carried by the cross bar to a position where they jam the conveyer.